This invention relates to electromagnetic patch antennas excited by dual feeds for generation of circularly polarized radiation and, more particularly, to the inclusion of a capacitive block between radiator and ground plane to balance asymmetry in locations of the feeds to introduce uniformity to the circularly polarized radiation.
Patch antennas may be constructed individually or in arrays of patch radiators operating with a common ground plane for transmitting and receiving beams of electromagnetic radiation in a wide variety of situations including communication and radar. A patch antenna is suitable for use both in fixed and mobile installations. The light weight of a patch antenna enhances the suitability of the antenna for use in the construction of an antenna system to be carried by a satellite encircling the earth.
In the usual construction of a patch antenna, one or more disc-shaped radiators are positioned in front of a ground-plane element, and spaced apart from the ground-plane element to permit radiation from the radiators. By way of example, the radiators and the ground-plane element may be formed of electrically-conductive sheets such as thin layers of brass, copper, aluminum or other electrically conductive material, and the sheets may be spaced apart by a layer of dielectric material. While patch antennas may be employed for the generation of radiation with linear, circular or elliptical polarization, the generation of circularly polarized radiation is of particular interest herein. Circularly polarized radiation is obtained frequently by the use of two electromagnetic feeds located on a ground-plane element beneath a radiator, and located ninety degrees apart around a central axis of a radiator in a space-quadrature relationship. The two feeds are excited with signals having sinusoidal waveforms which are ninety degrees out of phase, this being phase quadrature. Two well-known forms of feed are slots located in the ground plane element and extending both beneath and slightly beyond the radiator, and posts which pass through apertures in the ground-plane element to extend partway to the radiator.
A problem arises in the foregoing arrangement of feeds. It is noted that each of the feeds is excited independently of the other, and that the resulting circular polarization of emitted radiation results from a summation of two linearly polarized waves excited by respective ones of the feeds. Since the two feeds are located off to one side of the center of the radiator, the spacing between the feeds is sufficiently close to induce mutual coupling between the feeds. As a result of the mutual coupling, the circularly polarized radiation, which ideally has transverse components of equal magnitude, becomes slightly elliptical. As a result, the intensity of radiation varies with direction about a central axis of the radiator. This variation may introduce excessive signal loss, particularly in the case of reception of weak signals.
In the foregoing description, and in the ensuing description of the patch antennas, reference is made to the transmission of radiation. However, it is to be understood that the patch antennas discussed herein operate in reciprocal fashion such that the radiation characteristics are the same for both transmission and reception.